Casting slurry for the production of shell molds

ABSTRACT

A casting slurry for producing shell molds for casting parts includes a metal alloy, the slurry includes powder particles and a binder, the binder includes colloidal yttrium oxide, and the powder particles includes calcia-stabilized zirconia.

TECHNICAL FIELD

The present disclosure relates to the field of casting, in particular to investment (or lost-wax) casting processes, and more particularly to the slurries used in such processes, notably for the production of casting shell molds.

PRIOR ART

Investment (or lost-wax or lost-mold) casting processes have been known, in and of themselves, since antiquity. Such a process is for example described in the document FR3031921. They are particularly suitable for the production of metal parts with complex shapes. Investment casting is used for the production of turbomachine blades or impeller sectors, for example. In investment casting, the first step is normally the production of a shell mold, which generally consists in creating a model made of a comparatively low melting point material, such as wax or resin, around which a refractory material shell is then made. After the model has been destroyed, most commonly by discharging the model material from within the shell mold, which gives these processes their name, a molten metal is poured into this mold in order to fill the cavity formed by the model inside the mold after its discharge. Once the metal cools and solidifies, the mold can be opened or destroyed to recover a metal part that conforms to the shape of the model.

To make the carapace mold, the wax model is generally dipped in a casting slurry, then coated with sands and dried. These operations can be repeated in order to form several layers and to obtain the desired thickness and mechanical strength of the shell mold. However, the first layer of slurry used, called contact slurry, plays a key role in the quality of the cast metal parts. Indeed, this contact slurry allows the internal surface of the shell mold to form, coming directly in contact with the metal of the metal part to be molded.

In the aeronautical field, the production of parts such as turbine blades uses these investment casting processes. In particular, intermetallic alloys based on titanium aluminide (TiAl), due to their low density, are frequently used to produce these blades. This type of alloy has the particular feature of reacting easily with the constituents of the shell mold, the contact of this shell mold with the metal of the part being able to damage the surface finish of this part. To limit this effect, it is known to use a contact slurry including an yttrium oxide powder and a binder comprising colloidal yttrium oxide. However, this slurry has the disadvantage of being unstable. Indeed, contact slurry with this composition tends to gel quickly, after a few hours, for example after three or four hours. This disadvantage limits the industrial application of this type of slurry. Moreover, this type of slurry is expensive.

Alternatively, certain additives could be used, but none of these additives was satisfactory insofar as the improvement of one parameter of the slurry was compensated by the unacceptable regression of another parameter.

There is therefore a need for a new type of contact slurry, with increased stability over time.

DISCLOSURE OF THE INVENTION

The present disclosure relates to a casting slurry for producing shell molds for casting parts comprising a metal alloy, the slurry comprising powder particles and a binder, the binder comprising colloidal yttrium oxide, and the powder particles comprising calcia-stabilized zirconia.

A casting slurry is a slurry suitable for use in the formation of a shell mold into which molten metal will be poured. In particular, unlike any suspension, such a slurry comprises a binder, i.e., a compound ensuring cohesion between the powder particles and imparting mechanical strength to the shell mold during and after sintering. The binder can be inorganic. Conventionally, the powder particles can be sand particles (also known as “flour”), in particular refractory particles, generally having a diameter comprised between 1 micrometer and 100 micrometers.

The casting slurry used in the present disclosure comprises a binder comprising colloidal yttrium oxide, and powder particles comprising zirconia.

Surprisingly, it was observed by the inventors that the presence of calcia-stabilized zirconia (CSZ) in the powder particles significantly stabilized a slurry comprising yttrium oxide, and maintained sufficient fluidity, i.e., low viscosity. Conversely, the viscosity of a slurry of the prior art (for example a binder comprising colloidal yttrium oxide and powder particles comprising yttrium oxide) not having the composition of the present disclosure, i.e., a binder comprising colloidal yttrium oxide and powder particles comprising calcia-stabilized zirconia (CSZ), tends to increase over time, resulting in gelling of the slurry.

In the slurry of the present disclosure, the use of calcia-stabilized zirconia modifies the interaction between the binder and the powder particles to stabilize the slurry, while maintaining low reactivity with the metals to be molded, such as titanium aluminide (TiAl) alloys, and even lower reactivity than a slurry including an yttrium oxide powder and a binder comprising colloidal yttrium oxide. The slurry thus obtained has a longer life and can be reused. The baths used can also be larger, without leading to loss.

In some embodiments, the slurry is a contact slurry configured to come into contact with the metal of the part to be molded.

The first slurry used, which comes into direct contact with the metal of the part at the time of molding, is called contact slurry, as opposed to subsequent slurries, which are called reinforcement slurries and cover the previous layers of the shell mold being formed. A contact slurry is configured to conform to the shape of the part and not alter it. A contact slurry is often retained for longer periods of time than a reinforcement slurry which is consumed more quickly, whence an increased need for stability in a contact slurry. The slurry according to the present disclosure is therefore particularly suitable for use as a contact slurry, due to its stability over time and its non-reactivity with certain metals such as TiAl.

In certain embodiments, the mass content of calcium oxide in the calcia-stabilized zirconia is comprised between 1% and 30%, preferably between 3% and 20%, more preferably between 5% and 10%.

In certain embodiments, a mass ratio of the calcia-stabilized zirconia in the slurry is comprised between 65% and 75%, preferably between 68% and 72%, more preferably equal to 70%.

In certain embodiments, a mass ratio of the binder in the slurry is comprised between 20% and 40%, preferably between 25% and 35%, more preferably equal to 29.8%.

In certain embodiments, a mass ratio of additives in the slurry is less than 10%, preferably between 0.1% and 5%, more preferably between 0.5% and 2%.

In certain embodiments, the viscosity of the slurry is comprised between 0.1 and 2 Pa.s.

More precisely, the viscosity of the slurry is maintained at a value comprised between 0.1 and 2 Pa.s for a period of at least 24 hours. In particular, these values facilitate the accessibility of the slurry to certain narrow zones of the model.

In certain embodiments, the casting slurry is configured for the production of shell molds for casting parts comprising a titanium aluminide-based metal alloy.

The slurry according to the present disclosure is particularly suitable for use as a contact slurry, due to its stability over time and its non-reactivity with titanium aluminide (TiAl)-based metal alloys.

The present disclosure also relates to the use of a casting slurry in accordance with any one of the preceding embodiments for the production of a shell mold.

The present disclosure also relates to a process for producing a shell mold for casting parts, the process comprising the steps of:

-   -   providing a model of a part to be produced;     -   dipping the model in a contact slurry in accordance with any of         the preceding embodiments;     -   sandblasting the dipped model in a contact sand comprising         yttrium oxide;     -   drying the layer obtained by the preceding steps;     -   dipping the model in a reinforcement slurry, sandblasting the         dipped model in a reinforcement sand, and drying the layer         obtained, until a desired shell mold thickness is obtained;     -   removing the part model.

In certain embodiments, the reinforcement slurry comprises a binder and powder particles, the binder being selected from: ethyl silicate, sodium silicate or colloids including, in particular, colloidal silica, colloidal alumina, colloidal yttrium oxide or colloidal zirconia.

In certain embodiments, the powder particles comprise at least one compound among alumina, mullite, zirconia, mullite-zirconia composites.

The present disclosure also relates to a shell mold obtained by a process in accordance with any one of the preceding embodiments.

The shell mold obtained by the process according to the present disclosure limits the oxygen-rich reaction layer that forms on the surface of a metal part, such as an aeronautical engine blade, cast in this shell mold. The reaction layer is defined here as the thickness at which the oxygen concentration is greater than at least twice the concentration measured in the base alloy. In particular, for an isothermal contact at 1600° C. for a duration of 5 min, this reaction layer remains less than 15 μm for the part thus obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages will be better understood upon reading the detailed description below of various embodiments of the invention given by way of non-limiting examples. This description refers to the appended pages of figures, wherein:

FIG. 1 schematically represents the steps of a process for producing a shell mold for casting processes;

FIG. 2 is a graph showing change in the viscosity of a control slurry, and of the slurry of the present disclosure, as a function of shear stress.

DESCRIPTION OF THE EMBODIMENTS

The process for producing aeronautical parts, in particular a turbine blade or a turbine blade cluster, is a casting process. The various steps of this process are described for example in the document FR3031921.

The first step of this process consists in creating a wax cluster model, also called ‘non-permanent cluster’. In a second step, the shell mold is made from the wax cluster. At the end of this operation, the wax constituting the cluster model is removed from the mold. This wax removal is done by heating the shell mold in an autoclave (or the like) at a temperature greater than the melting temperature of the wax. In a third step, the metal blade cluster is formed in the shell mold by pouring molten metal into the shell mold. In a fourth step, after the metal has cooled and solidified in the shell mold, the cluster is removed from the shell mold. Finally, in a fifth step, each of the blades is separated from the rest of the cluster and finished by finishing processes such as machining.

The invention relates in particular to the production of the shell mold in which the metal casting will be carried out, and more specifically to the contact slurry used for the production of this mold. The various steps of this process are illustrated in FIG. 1.

The first step (step S1) comprises providing a model made of wax, or other equivalent material that can be easily discharged later, of the part. In a second step, the wax model is dipped into a first slurry, the contact slurry (step S2), comprising powder particles and a binder. Sandblasting, i.e., deposition of sand particles called contact stucco, is then carried out, followed by a drying of the layer obtained (step S3). This sandblasting step reinforces the layer and facilitates the adhesion of the next layer.

The layer thus obtained is then dipped in a second slurry, called reinforcement slurry (step S4). A deposition of sand particles, called reinforcement stucco, is then carried out, followed by a drying of the layer obtained (step S5). Steps S4 and S5 are repeated N times, until a determined thickness of shell mold is obtained. Finally, when the desired thickness is reached, a dewaxing step, consisting of removing the wax model from the model, followed by heat treatment, is performed (step S6). After removal of the wax model, a ceramic shell mold whose cavity is a negative reproduction of all the details of the part to be molded is obtained. The heat treatment includes the firing of the mold obtained, the firing temperature preferably being comprised between 1000 and 1200° C.

The slurries used are composed of particles of ceramic materials, in particular alumina, mullite, zirconia or others, with a mineral colloidal binder and, if need be, adjuvants such as wetting agents or antifoam agents.

In the context of the production of titanium aluminide (TiAl)-based aeronautical parts, the contact slurry used in step S2 comprises yttrium oxide.

The contact stucco used in step S3 may also comprise yttrium oxide. The reinforcement slurry and reinforcement stucco used in steps S4 and S5 may comprise mullite, alumina, silico-alumina, silica, zircon, zirconia or yttrium oxide, for example.

The invention relates more particularly to the contact slurry used in step S2, and in particular to the presence of colloidal yttrium oxide and calcia-stabilized zirconia (CSZ) in the powder particles therein.

In order to appreciate the influence of the presence of CSZ in a contact slurry, the inventors first studied a control slurry, denoted slurry A, intended to be used as a contact slurry for the production of a shell mold. Slurry A can have the following composition, expressed in percentages by mass:

-   -   binder (colloidal yttrium oxide): 24.5%;     -   powder particles (yttrium oxide powder): 75%;     -   wetting agent, antifoam agent and other additives: 0.5%.

This mass distribution is given here by way of example, it being understood that a variation in mass distribution of up to 10% is possible. Slurry A does not contain CSZ.

Furthermore, the inventors have studied a slurry B which the inventors have determined exhibits similar reactivity with TiAl as slurry A, and whose powder particles comprise calcia-stabilized zirconia (CSZ), with CaO acting as a stabilizing agent. CSZ can be obtained for example by reactive sintering. The CaO content in mass percentage in the powder is comprised between 1% and 20% by weight. The slurry B thus obtained has the following mass percentages:

-   -   binder (colloidal yttrium oxide): 29.8%;     -   powder particles (CSZ): 70%, including 5% CaO;     -   wetting agent, antifoam agent and other additives: 0.2%.

Similarly, this mass distribution is given here by way of example, it being understood that a variation of the mass distribution is possible in the ranges previously mentioned.

Slurry B also includes unavoidable impurities. Among unavoidable impurities, for example, mention may be made of silicon dioxide (SiO₂), titanium dioxide (TiO₂), iron oxide (Fe₂O₃) or alumina (Al₂O₃). Unavoidable impurities are defined as those elements which are not intentionally added to the composition and which are brought in with other elements.

The curves shown in FIG. 2 illustrate the influence of the composition used for the contact slurry according to the present disclosure on its stability. This figure shows the change in the dynamic viscosity η in Pa.s of the slurry, as a function of a shear applied to this slurry. These measurements are performed using a rotary rheometer with coaxial cylindrical geometries, to apply to the slurry a shear comprised between 0.1 and 100 s⁻¹. More precisely, the dynamic viscosity η can be calculated in a non-normalized manner from the shear stress T and a shear rate Ý, according to the relationship η=τ/Ý. Curve (a) represents the viscosity of slurry A after 0.5 h, curve (b) represents the viscosity of slurry A after 2 h, curve (c) represents the viscosity of slurry A after 3.5 h, and curve (d) represents the viscosity of slurry B of the invention after 24 h. The above-mentioned times are determined from a time t0 corresponding to the end of the production of the slurry.

Curves (a) and (b) illustrating the viscosity of slurry A after 0.5 h and after 2 h are substantially coincident. For a low shear, of the order of 0.1 s⁻¹, the viscosity of slurry A is roughly equal to 4 Pa.s after 2 h. This viscosity then increases very rapidly with time, and reaches a value greater than 25 Pa.s after 3.5 h. In other words, the slurry quickly becomes very viscous, and tends to gel.

Conversely, curve (d) illustrating the viscosity of slurry B of the invention shows that the viscosity of slurry B remains less than 1 Pa.s after 24 h, regardless of the shear applied thereto. Thus, slurry B has increased stability compared with slurry A, and remains fluid by maintaining a low viscosity even 24 h after preparation of this slurry. Furthermore, the composition of slurry B maintains a low reactivity with TiAl alloys, equivalent or even lower than that of slurry A.

Although the present invention has been described with reference to specific example embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated/mentioned embodiments may be combined in additional embodiments. Consequently, the description and drawings should be considered in an illustrative rather than restrictive sense.

It is also obvious that all the features described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the features described with reference to a device are transposable, alone or in combination, to a process. 

1. A casting slurry for producing shell molds for casting parts comprising a metal alloy, the slurry comprising powder particles and a binder, characterized in that the binder comprises colloidal yttrium oxide, and in that the powder particles comprise calcia-stabilized zirconia, a mass ratio of the calcia-stabilized zirconia in the slurry being comprised between 65% and 75%.
 2. The slurry as claimed in claim 1, the slurry being a contact slurry configured to come into contact with the metal of the part to be molded.
 3. The slurry as claimed in claim 1, wherein the mass content of calcium oxide in the calcia-stabilized zirconia is comprised between 1% and 20%.
 4. The slurry as claimed in claim 1, wherein the viscosity of the slurry is comprised between 0.1 and 2 Pa.s.
 5. The casting slurry as claimed in claim 1, configured for the production of shell molds for casting parts comprising a titanium aluminide-based metal alloy.
 6. Use of the casting slurry as claimed in claim 1 for the production of a shell mold.
 7. A process for producing a shell mold for casting parts, the process comprising the steps of: providing a model of a part to be produced; dipping the model in a contact slurry as claimed in claim 1; sandblasting the dipped model in a contact sand comprising yttrium oxide; drying the layer obtained by the preceding steps; dipping the model in a reinforcement slurry, sandblasting the model dipped in a reinforcement sand, and drying the layer obtained, until a desired shell mold thickness is obtained; removing the part model.
 8. A shell mold obtained by the process as claimed in claim
 7. 9. The slurry as claimed in claim 1, wherein the mass ratio of the calcia-stabilized zirconia in the slurry is between 68% and 72%.
 10. The slurry as claimed in claim 1, wherein the mass ratio of the calcia-stabilized zirconia in the slurry is equal to 70%. 